This invention relates generally to ballasts used to power gas discharge lamps. More particularly, this invention pertains to circuits used in conjunction with a magnetic ballast to ignite a gas discharge lamp.
Gas discharge lamps require a high voltage pulse of electricity for ignition. The design of the lamp determines the voltage requirements for the ignition pulse, and there is typically a minimum and maximum voltage requirement for the ignition pulse. After a gas discharge lamp is ignited, the lamp presents a negative resistance. Therefore, a ballast is used to control and limit the amount of current going to the lamp after ignition. In many commercial lighting environments, the ballast and ignition circuit (sometimes referred to as a “starter” circuit) are connected to the lamp using electrical wires placed in a conduit. This arrangement creates a parasitic capacitance which increases with increased conduit length. The larger the parasitic capacitance, the greater the load affecting the amplitude of an ignition pulse from a lamp starter circuit. The conduit length actually installed in the field is variable, so the amount of parasitic capacitance associated with the conduit is variable. A starter circuit which can simply and reliably provide ignition pulses having a voltage within the prescribed range over a wide variety of conduit lengths is desirable.
Many circuits have been developed to deliver ignition pulses to lamps over varying starter-circuit-to-lamp conduit lengths. For example, U.S. Pat. No. 6,522,088 describes a starter circuit having a voltage clamping device connected between the two leads to the lamp. The ballast circuitry is capable of generating an ignition pulse having a voltage in excess of the prescribed range for the lamp. Due to the higher voltage of the ignition pulse, a longer conduit length between the lamp and the ballast circuitry is possible. If the longer length is used, the parasitic capacitance reduces the voltage of the ignition pulse to within the prescribed range. The voltage clamping device has an impedance which varies with voltage such that if the voltage exceeds the clamping voltage, the impedance drops and thereby lowers the voltage of the ignition pulse delivered to the lamp. The voltage clamping device is typically comprised of two varistors connected in series wherein the combined clamping voltage of the two varistors is near the maximum voltage acceptable for the lamp. Unfortunately, using a clamping device in the starting circuit adds cost which is disadvantageous in the highly competitive lighting industry. Also, the clamping device may be required to dissipate significant energy when clamping high voltage ignition pulses. This decreases reliability of the device.
Publication No. JP2005251722 describes a device having a second starting device positioned close to the lamp when the conduit length between the first starting device and the lamp is long. When the conduit between the first starting device and the lamp is short, a second starting device is not used. This provides for a wider range of acceptable conduit lengths between the first starter device and the lamp.
U.S. Pat. No. 6,396,220 describes circuitry with a first and a second reactive energy source. The first reactive energy source generates ignition pulses for longer conduit lengths, and the second reactive energy source generates ignition pulses for shorter conduit lengths. A switch is provided so that either the first or the second reactive energy source is utilized. There are several embodiments wherein different components of the ignition circuitry are switched on and off, but in all embodiments a switch is used to select between components which generate ignition pulses having different voltages.
In the highly competitive field of lighting electronics cost and reliability are important considerations. Costs can be reduced by using fewer components and/or using components designed for lower voltages. Also, as a general rule, the fewer components used, the more reliable the system. Therefore, a system using fewer components and/or components designed for lower voltage is preferred.